Sigma-delta analog-to-digital converters ("ADCs") and DACs have recently come into widespread use with the development of suitable process technology and the increase in digital audio and other applications. Sigma-delta converters utilize oversampling techniques (i.e., sampling at rates greater than the Nyquist rate) to achieve high signal-to-noise ratios. Such converters also exhibit excellent linearity. Additionally, sigma-delta converters are relatively straight-forward and inexpensive to implement due to their simplicity.
A sigma-delta ADC converts an analog input voltage to digital output samples at a predetermined rate. A typical sigma-delta ADC includes a front-end modulator which, by performing an oversampling technique referred to as "noise shaping", manipulates the noise spectrum signal such that a major component of the quantization noise power is shifted to a frequency range higher than the upper frequency limit of the band of interest, which is typically the signal bandwidth (within the output samples). Subsequent filtering ("decimation") is performed in the digital domain to reduce the high frequency quantization noise component of the digital output samples.
A conventional, first order sigma-delta ADC is shown in block diagram form in FIG. 1. The ADC includes a modulator 50 and a decimator 59. Modulator 50 converts an input voltage Vin, received on line 70, into a one-bit data stream X at a rate determined by the sampling frequency Kf.sub.S. Modulator 50 performs oversampling and noise shaping on the input voltage. The one-bit data stream X is provided on line 57 to decimator 59 which low-pass filters the data stream to reduce the quantization noise component thereof, and provides filtered N-bit output samples at a rate f.sub.S on output line(s) 61. In short, the decimator 59 decimates, by a factor K, the one-bit data stream X.
The modulator 50 includes an input circuit 51 which samples the input signal Vin and provides the sampled input signal to a summing circuit 54. Summing circuit 54 subtracts a feedback signal (described below), received on line 65, from the sampled input signal and provides the output signal difference on line 55 to an integrator 56. Integrator 56 conventionally "integrates" the output signal difference from the summing circuit 55 and provides an output signal A to a clocked, latched comparator 58. Summing circuit 54 can generally be considered as an input section of the integrator. Comparator 58 conventionally is clocked at a rate Kf.sub.s by a clock signal applied to line 58A. For each clock pulse, comparator 58 "compares" signal A to ground and provides a one-bit output. The sequence of outputs for a sequence of clock pulses form on line 57 the one-bit data stream output X of the modulator. Thus, comparator 58 is effectively a one-bit ADC.
Data stream X is also provided on feedback line 63 to a feedback circuit 52, which includes a one-bit DAC (not shown). The stream X controls a switch 53 within the feedback circuit 52 such that either a positive feedback reference voltage +Vref, received on line 74, or a negative feedback reference voltage -Vref, received on line 76, will be applied, for each bit in stream X, via feedback line 65, to circuitry 66, which generates and supplies a corresponding feedback signal to the summing circuit 54. This switching operation of the feedback circuit 52 of the modulator 50 is conventional for a closed-loop circuit and should be readily understood by those skilled in the art.
Sigma-delta modulators are typically implemented with switched-capacitor circuits. While the operation of the modulator was generally described above in terms of delivering signals, in a switched-capacitor implementation, the signals are packets of charge. Thus, a charge packet is developed each time the input signal is sampled and each time a feedback signal is generated, with such charges or the net charge being "transferred" from the input and feedback circuits to the integrator via the summing circuit.
In a switched-capacitor implementation, the integrator 50 typically includes an op amp and a feedback-connected integrator capacitor (not shown in FIG. 1). The input and feedback circuits typically include switches and capacitors for respectively sampling the input and feedback reference voltages. During operation, an input capacitor is typically charged by a voltage source (i.e., the input voltage source or feedback reference voltage source) through a first switch (or switches) during a first time interval (clock phase), and charge is thereafter "transferred" through a second switch (or switches) from the input capacitor to the feedback-connected integrator capacitor during a second, non-overlapping time interval. The switches often include CMOS transistors due to their high performance and yield.
It should be appreciated by those skilled in the art that, as used in the art and herein, charge "transfer" refers to a charging of the integrator capacitor C3 by the integrator output voltage to compensate for charging (sampling) of the input capacitor C1 due to the equipotential surface between one plate of the input capacitor C1 and one plate of the integrator capacitor C3 (see FIG. 2). Thus, a literal, physical movement of charge from the input capacitor to the integrator capacitor may not occur.
The above description of the modulator, in which a single input voltage is sampled (with respect to ground), assumes the integrator 11 based on use of a single-ended op amp. As will be understood by those skilled in the art, however, the integrator may include a differential amplifier for which positive and negative input voltages are separately sampled.
Whether single-ended or differential, CMOS op amps typically have associated input offset voltages within the range of 1-10 mv (whereas ideally the input offset voltage should be 0). During operation, the difference between the voltages on the input terminals of a differential op amp will be equal to the input offset voltage, when the output voltage is at 0 volts.
In sigma-delta ADC modulators, the input offset voltage of the op amp may charge the feedback circuit capacitor(s) (i.e., the offset voltage is sampled by the feedback circuit) and that charge may be "transferred" to the integrator. Ideally, the only charge transferred from the feedback circuit should be that due to the sampled feedback reference voltages. As will be appreciated by those skilled in the art, the amount of charge transferred from the feedback circuit to the integrator is typically controlled by the transition density (i.e., the number of transitions from 1 to 0 or from 0 to 1) of the digital output signal (X in FIG. 1) of the modulator. Because the transition density of the digital output signal is typically non-linear, the transfer of charge resulting from sampling the input offset voltage is also non-linear. As a result of the non-linear transfer of charge due to the sampling of the input offset voltage, such modulators may suffer from integral non-linearity performance inaccuracies and encounter repetitive noise patterns known as "idle tones". The following is a detailed discussion of such prior art modulator performance.
Referring to FIG. 2, where like elements are referred to by identical reference characters to those of FIG. 1, a prior art switched-capacitor DAC system is shown including a DAC 52 and an integrator 56. During operation, the DAC 52 samples the reference voltage Vref and transfers charge corresponding to +Vref or -Vref to the integrator 56. The reference voltage Vref can be considered an input voltage to the DAC.
Integrator 56 includes an op amp 60, a first integrator capacitor C3 connected between non-inverting output lead 66 and inverting input lead 62 of the op amp, and a second integrator capacitor C4 connected between inverting output lead 68 and non-inverting input lead 64.
The switched-capacitor DAC 52 includes input lines 74 and 76 respectively receiving the positive and the negative terminals of the reference voltage Vref. A first input capacitor C1 is coupled to the input lines 74 and 76 through a first switching circuit 78 and to the input leads of the op amp through a second switching circuit 80. A second input capacitor C2 is similarly coupled to the input lines 74 and 76 through the first switching circuit 78 and to the input leads 62 and 64 of the op amp through the second switching circuit 80. Capacitors C1 and C2 sample (i.e., are charged by) the reference voltage Vref through switching circuit 78 and transfer charge to capacitors C3 and C4 through switching circuit 80. The values of capacitors C1 and C2 are preferably equal as are the values of capacitors C3 and C4.
Switching circuit 78 includes switches labeled with the symbol P1 or P2. The switches labeled P1 are controlled by the control signal P1 and the switches labeled P2 are controlled by the control signal P2 (see FIG. 3). Switching circuit 80 includes switches labeled with the symbol R1 or R2. The switches labeled R1 are controlled by the control signal R1 and the switches labeled R2 are controlled by the control signal R2 (see FIG. 3).
Shown in the timing diagram of FIG. 3 are the control signals P1, P2, R1 and R2 as well as the digital input signal Y used to generate signals R1 and R2. In a sigma-delta ADC modulator application, signal Y is typically the digital output signal (i.e., signal X in FIG. 1). The control signals are shown on the same time axis and the vertical placement of one above the other does not signify that one attains different voltage levels than the other; the "high" and "low" voltage levels of the signals are relative to each other only. As is conventional for a switched-capacitor circuit, the P1 and P2 switches operate in two non-overlapping time intervals (or clock phases). During interval 1 (marked on the time axis), signal P1 is at a high voltage level and signal P2 is at a low voltage level. During interval 2, signal P1 is low and signal P2 is high.
Signal P1 controls the switches labeled P1 such that during interval 1 (when P1 is high), the P1 labeled switches are closed (and conduct current) and, during interval 2 (when P1 is low), the P1 labeled switches are open (and prevent the flow of current). Conversely, the switches labeled P2 are open during interval 1 and are closed during interval 2. It is important that the signals P1 and P2 are not high at the same time so that accurate reference voltage sampling occurs. Thus, as will be understood by those skilled in the art, the circuit (not shown) generating signals P1 and P2 typically establishes a "break-before-make" operation to ensure that the control signals are not simultaneously high.
The control signals R1 and R2 depend on the level of the signal Y in accordance with the following relationships. R1=P1.multidot.YB+P2.multidot.Y and R2=P1.multidot.Y+P2.multidot.YB, where YB is the complement of Y, "+" represents a logical OR operation and ".multidot." represents a logical AND operation. When Y is low, signal R1 is identical to signal P1 and signal R2 is identical to signal P2. When Y is high, signal R1 is identical to signal P2 and signal R2 is identical to signal P1.
Referring back to FIG. 2, switching circuit 78 includes a first switch S1 connected between input line 74 and the left plate of capacitor C1, a second switch S2 connected between input line 76 and the left plate of capacitor C1, a third switch S3 connected between input line 76 and the left plate of capacitor C2, and a fourth switch S4 connected between input line 74 and the left plate of capacitor C2. Switching circuit 80 includes a first switch S5 connected between the right plate of capacitor C1 and the inverting input lead 62 of op amp 60, a second switch S6 connected between the right plate of capacitor C1 and the non-inverting input lead 64, a third switch S7 connected between the right plate of capacitor C2 and the non-inverting input lead 64, and a fourth switch S8 connected between the right plate of capacitor of C2 and the inverting input lead 62. As labeled, switches S1 and S3 are controlled by control signal P1, switches S2 and S4 are controlled by control signal P2, switches S6 and S8 are controlled by control signal R1, and switches S5 and S7 are controlled by control signal R2.
As should be readily understood by those skilled in the art, the input capacitors C1 and C2 operate to sample the reference voltages through switching circuit 78 and to transfer charge to the integrating capacitors C3 and C4 through the switching circuit 80. The cross-coupled arrangement of the input capacitors enables the reference voltages to be sampled during both time intervals and charge to be transferred during both time intervals. The timing diagram of FIG. 3 assumes that signal Y transitions once during intervals 1 and 2 and that the transition occurs between interval 2 and interval 1.
While the prior DAC of FIG. 2 operates effectively to sample the reference voltages during both time intervals and to transfer charge to the integrating capacitors during both time intervals, the DAC may produce integral non-linearity errors due to the presence of an op amp input offset voltage. FIG. 4 schematically illustrates the prior art DAC of FIG. 2 for the situation where the reference voltages are equal to 0 volts (i.e., the input lines 74 and 76 are tied to ground) and the op amp 70 has an associated input offset voltage V.sub.OS, so that the effect of the input offset voltage can be more easily analyzed.
Such an analysis follows.
During interval 1, assuming signal Y is low, the left plate of capacitor C1 is connected to ground through switch S1 and the right plate of capacitor C1 is connected through switch S6 to the non-inverting input lead 64 which is at a voltage level of -V.sub.OS /2. [The common-mode voltage of the input leads of the op amp is assumed to be 0 volts. Therefore, the inverting input lead 62 is at a voltage level of +V.sub.OS /2 and the non-inverting input lead 64 is at a voltage level of -V.sub.OS /2.] During the immediately following interval 2, assuming that Y does not transition from low to high, the left plate of capacitor C1 is connected to ground through switch S2 and the right plate of capacitor C1 is connected through switch S5 to inverting input lead 62 (which is at a voltage level of V.sub.OS /2). Thus, a total charge (related to the change in voltage on the right plate of capacitor C1) of C1.multidot.V.sub.OS has accumulated on capacitor C1 during the two intervals and that same charge of C1.multidot.V.sub.OS is transferred to capacitor C3. As will be appreciated by those skilled in the art, an equal and opposite charge will be transferred from capacitor C2 to capacitor C4 due to the differential arrangement of the circuitry and the capacitors C1 and C2 being of equal value. Therefore, the charging of capacitor C2 and the transfer of charge from capacitor C2 to C4 will not be separately analyzed. In sum, if signal Y remains low (does not transition), a charge equal to C1.multidot.V.sub.OS is transferred to the integrator capacitor C3 during the second of two consecutive intervals.
However, when signal Y transitions between two consecutive intervals, the right hand plate of the input capacitor remains connected to the same node and no charge is transferred, as described below. During interval 2 (assuming Y is low), the left plate of capacitor C1 is connected through switch S2 to ground and the right plate of capacitor C1 is connected through switch S5 to the inverting input lead 62 (at a voltage level of V.sub.OS /2). During the subsequent interval 1 (assuming Y transitions to high), the left plate of capacitor C1 is connected to ground through switch S1 and the right plate of capacitor C1 remains connected through switch S5 to the inverting input lead 62. Thus, the voltage on the right plate of capacitor C1 does not change between intervals 2 and 1 and no charge is transferred. In sum, if Y does not transition between two consecutive time intervals, then a charge of Cl.multidot.V.sub.OS is transferred and if Y does transition between the time intervals, then no charge is transferred.
The total charge transferred due to the input offset voltage of the op amp over a period of time, therefore, depends on the density of transitions in the signal Y. In a sigma-delta ADC modulator, the transition density in the output signal (Y) is typically non-linear. FIG. 5 is a graph illustrating the transition density for a digital output signal of a second-order sigma-delta ADC modulator over an analog input voltage range of -1 volt to +1 volt. As shown, the transition density is approximately equal to 70 percent for mid-scale code (i.e., 0 volts) and is approximately equal to 25 percent for positive and negative full-scale code (i.e., .+-.1 volts). As a result, a sigma-delta ADC modulator employing the prior art DAC of FIG. 2 may encounter integral non-linearity accuracy problems and idle tones.
Accordingly, a general object of the present invention is to provide a high-performance switched-capacitor DAC including circuitry for reducing op amp offset voltage non-linearity errors.
Other objects and advantages will be apparent from the detailed description below.